Cardiac contraction during a heartbeat causes a time dependent flow of blood in the arteries of the body. The flow of blood in a single beat is pulse-like and caused by a rise in the pressure of the blood from a static value, called the diastolic pressure, to a maximum value at the peak of the pulse, called the systolic pressure. The pulse-like variation in blood pressure induces a corresponding pulse-like variation in the refractive index of the blood plasma. That is, the variation of blood pressure in a heart beat induces a variation in the optical properties of the blood.
Conventional blood pressure monitoring devices typically involve measuring blood pressure with an inflatable cuff, whether automated, or by manual inflation. In these arrangements, the cuff is placed around a patient's arm, directly over an artery. The cuff inflates in order to compress the underlying artery of the patient, and cut off the circulation to the arm. The cuff then deflates slowly and releases pressure on the arm and allows for the determination of systolic and diastolic readings by direct observation or measurement of Korotkoff sounds. The most common manual and automated blood pressure monitors involve inflating and deflating a cuff and is both time consuming and involves some discomfort for the patient.
Continuous measurement of blood pressure by means of optically based pressure transducer tips, inserted into the blood stream using a catheter, have been proposed, as illustrated in U.S. Pat. No. 3,249,105 to Polyanyi, and Franke U.S. Pat. No. 3,215,135, and more recently, to Sawatari et al, U.S. Pat. No. 5,987,995. While these techniques provide a means of measuring blood pressure continuously, and are useful in operative settings, they are invasive, expensive, require medical staff to administer, and are not designed for continuous every day operations.
Non-invasive blood pressure measurement systems include a tissue contact stress sensing system involving measurement of tonometric blood pressure non-invasively using a one-dimensional optical sensor array, as described by U.S. Pat. No. 5,158,091 to Butterfield et al. This method measures a pressure waveform on the skin tissue, which is not the same as the pressure on the artery walls, and therefore necessitates calibration. Calibration is extremely difficult for this type of sensor, and therefore is not expected to provide accurate or reproducible readings.
Recent inventions have introduced non invasive measurement of systolic and diastolic pressure through optical pulse train, and signal processing means. These inventions involve continuous measurement of blood pressure as disclosed in the prior art by U.S. Pat. No. 7,179,228 to Banet. While this technique provides a means of measuring blood pressure in a cuff-less, non invasive setting, it requires prior calibration by the patient, and does not provide the same level of accuracy as other methods.
The abovementioned prior art lacks the advantages of a continuous blood pressure monitoring system that is capable of being worn for continual periods, whilst offering the patient complete mobility. Thus, it is the object of various embodiments of this invention, to provide a means of measuring blood pressure on a continuous or non-continuous basis, using a wearable, cuff-less, non-invasive method, for every day use. It is a further object of various embodiments of the invention to provide a mobile system that transmits patient vitals on a continuous basis using a wireless sensor network. The advantage of such a system is that it can be used in any setting requiring continuous vital signs measurement in a mobile environment whether it be monitoring of soldier's vital signs on the battlefield, in a homecare environment on a daily basis where mobility is critical, or even by astronauts on board a space craft or station.